1. Field of the Invention
This invention relates to a magnetic reproducing apparatus, and more particularly to a magnetic reproducing apparatus adapted for converting, e.g., a video signal to a digital signal to reproduce a signal recorded on a magnetic recording medium by making use of so called a partial response system.
2. Description of the Prior Art
Generally, in magnetic recording/reproduction, in order to compensate an amplitude distortion or a phase distortion, etc. resulting from the recording/reproducing characteristic with respect to a magnetic recording medium, an equalizer is used. In recent years, an adaptive equalizing system as used in communication has been adopted also in such magnetic recording/reproduction.
The adaptive equalizing has been conventionally developed as a technology to carry out high speed data transmission utilizing a telephone line. In the telephone line, the transmission path characteristic varies depending upon connection state of the line. For this reason, since the transmission path characteristic can not be sufficiently corrected with a fixed equalizer, there is the necessity of adaptively adjusting the characteristic of the equalizer.
In such a communication system, there are a method in which a signal of which waveform, etc. is known is transmitted to examine the transmission path characteristic to thereafter transmit a necessary signal (automatic equalizing), and there are methods in which a signal itself desired to be transmitted is used to examine the transmission path characteristic (adaptive equalizing). In either case, the object of the equalizer is to automatically eliminate distortion from a received signal waveform which was distorted by passing through a transmission path to faithfully restore a transmit signal waveform.
In order to consider application to the magnetic recording/reproduction of adaptive equalizing as described above, a digital VTR (Video Tape Recorder) adapted to convert a video and to a digital signal to record the digital signal onto a magnetic tape (video tape) for reproducing it therefrom by using the so-called partial response system is assumed, This partial response system is a system of shaping a spectrum of code by positively utilizing interference between codes due to the transfer characteristic of the transmission path (or recording medium). For example, NRZI code, Interleaved NRZI code, and the like belong to the partial response. On the recording side, there is provided a precoder to convert input data to an intermediate series in order to avoid propagation of code error at the time of reproduction (at the time of discrimination). An example of the configuration on the reproducing side in the case where the above-mentioned adaptive equalizing system is adopted for a digital VTR for carrying out magnetic recording/reproduction by making use of such a partial response system is shown in FIG. 7.
In FIG. 7, a magnetic signal recorded on a recording tape (not shown) is converted to an electric signal by using a magnetic head 101. The electric signal thus obtained is then is amplified by means of a reproducing amplifier 102. The signal thus amplified is sent to a detection characteristic circuit 103. This detection characteristic circuit 103 has a detection characteristic (encode characteristic) of the partial response, and a characteristic of (1+D) in the case of the class IV. An output signal from the detection characteristic circuit 103 is delivered to an equalizer 104 comprised of a FIR (Finite Impulse Response) or a transversal filter, at which adaptive equalizing processing is implemented to that output signal. The output signal thus processed is then delivered to a decoder 105, at which discrimination between "1" and "0" by level comparison (comparate), etc. is carried out. Thus, decoding of data series at the time of recording is conducted.
An output d from the decoder 105 is sent to an adder (error detector) 106, at which an output y from the equalizer 104 is subtracted from the output d. Thus, an (residual) error e is taken out. The error e thus obtained is sent to an adaptive control unit 107. To this adaptive control unit 107, an output x from the detection characteristic circuit 102 is delivered as so called a reference input. The adaptive control unit 107 adjusts the filter characteristic of the equalizer 104 so as to allow a signal power of the (residual) error to be minimum. In the case where so called a transversal filter is used as the equalizer 104, multiplication coefficients (tap coefficients) are adaptively modified (corrected) and updated. Thus, the characteristic of the transversal filter is adjusted so that a characteristic close to an inverse characteristic of the electromagnetic conversion characteristic in the magnetic recording/reproduction is provided.
An output from the encoder 105 is sent to a signal processing circuit 108, at which reproduction of a synchronous block and/or error correction, etc. are carried out. The signal thus processed is sent to a video signal processing circuit 109, at which restoring of original image data is carried out. In addition, although not shown, output data from the signal processing circuit 108 is sent to an audio signal processing circuit and a sub-code signal processing circuit, etc. In these circuits, respective processing are carried out.
Meanwhile, generally, at the time of building up of a servo i.e. when the operation of the capstan servo or the rotary drum servo is initiated, such as at the time of starting of reproduction of VTR, a regenerative signal becomes unstable. As a result, there are instances where updating of the adaptive filter coefficients is not correctly carried out for such an unstable signal, giving rise to a difficulty wherein the series of the tap coefficients diverge.
Further, generally, in VTR, etc., at the time of a variable speed reproduction when reproduction is carried out at a tape speed different from a tape speed at the time of recording, a locus such that the reproducing head traverses a plurality of tracks is depicted. For this reason, for a time period during which scanning is carried out at a position between tracks, or scanning is carried out on a track of which azimuth angle is different from that of the reproducing head, the reproduction level is remarkably lowered. As a result, if a signal at such a portion is used to carry out modifying/updating of the adaptive filter tap coefficients, the operation may frequently becomes unstable.
In addition, in a magnetic reproducing apparatus using rotary head, there is proposed a system in which reproduced data obtained without implementing accurate tracking onto recording tracks on a magnetic tape is written into a memory together with track addresses to read out, from this memory, data in accordance with the order of those track addresses. In this case, even at the time of a reproducing mode of an ordinary speed, a scanning locus of the reproducing head may traverse tracks. At this time, the production level is lowered to such an extent that no effective regenerative signal is provided. For this reason, if the adaptive equalizing is caused to be carried out, there is the possibility that the operation may become unstable.